Ligaments consist of bands of various forms, serving to connect together the articular extremities of bones, and are composed mainly of bundles of white fibrous tissue placed in parallel or closely interlaced with one another. A ligament is pliant and flexible, so as to allow freedom of movement, but strong, tough, and inextensile so as not to readily yield under severe force. Consequently, ligaments are well adapted to serve as the connecting medium between bones.
Tendons are white, glistening, fibrous cords, varying in length and thickness, of considerable strength, and devoid of elasticity. They are composed of white fibrous tissue, the fibrils of which have an undulating course parallel with each other and are firmly united together. Tendons are connected between muscles and movable structures such as bones, cartilages, ligaments and fibrous membranes.
The surgical treatment of tendon and ligament injuries is a significant therapeutic problem. Such injury can be profoundly disabling if left untreated. For example, it is well known that athletes' careers are often compromised or even terminated by injury to the cruciate ligaments of the knee. Even in non-athletes, injury to ligaments or tendons may affect any activity where walking or stair climbing is involved. Clinical evidence suggests that chronic instability from ligament damage results in early degenerative joint disease. Surgical reconstruction may be the best way of alleviating symptoms and preventing progressive long term damage. Although various remedial procedures have been developed, there is an unacceptably high incidence of fair or poor results.
Early reconstructive procedures for the cruciate deficient knee focused on manipulation of extra-articular (occurring outside of the joint) tissue to compensate for the loss of intra-articular cruciate. More recently, intra-articular repairs using tissues arising, transferred or transplanted within an individual, i.e., autogenous or autologous structure, have been employed.
The success of surgical reconstruction of damaged tendon or ligament tissue relies on the utilization of appropriate graft material. Optimally, such material should duplicate the original physiological function of the damaged tissue, be biocompatible, readily available, easy to implant, amenable to long term storage, free of transmittable disease and non-immunogenic.
Autologous tissue, including fascia lata, semitendinosus, patella tendon, and tissue from the iliotibial band, is one source of graft material presently employed in the reconstructive surgery of damaged connective tissue. Since the grafting material is taken from the patient, there is no risk of rejection. Although these autografts serve as good replacements, they are not without drawbacks. The limited supply of useable autogenic material is a major disadvantage. Moreover, considerable post-operative morbidity is related to dissection and sacrifice of host tissue, leaving the dissected tissue weaker than undisturbed tissues. Success rates are extremely variable both in the short term and after several years when many of the grafts elongate and result in recurrent instability.
There is a clear need for a non-autologous, readily available graft material which is mechanically adequate in both the long and short term. Short term success with such a graft is dependent upon biocompatibility, initial graft mechanical strength and the ability of the surgeon to implant the graft correctly with minimal post-operative trauma. For a graft to maintain stability in the long-term, it must be well-incorporated by the host and gradually remodeled into a structure with properties similar to the damaged tissue being replaced. Incorporation and remodeling usually involve, among other things, repopulation of the implant with the recipient's fibroblasts and some degree of revascularization.
Synthetic materials such as Goretex.RTM. have been developed as a substitute for connective tissue, but have shortcomings. Although synthetics are non-antigenic, constant wear may render them non-biocompatible due to host foreign body responses to wear debris. Moreover, mechanical mismatch between the synthetic material and the host may lead to failure of the implant. Finally, synthetics may not be well-incorporated into the recipient, i.e., they may not be repopulated by host fibroblasts, revascularized, or otherwise incorporated by the host tissues.
Allograft material, i.e., a graft of tissue between individuals of the same species but of disparate genotype, has also been employed, but may exhibit certain disadvantages. The supply of allograft material, like autologous tissue, is limited. There are also increasing concerns regarding viral contamination of allografts, whose viral progeny may ultimately proliferate and infect the recipient of the graft. A third drawback is the alloreactivity of allograft material. Indeed, hyperarcuate rejection can sometimes occur just hours after transplantation and is likely to be due to preformed antibodies to the graft antigens. Acute rejection represents the most commonly treated rejection event. Although many immunosuppressive agents are used to reduce alloreactivity, they can have serious side effects such as lowering the host's resistance to infection. One method being used to decrease the alloreactivity of allograft involves freezing or lyophilization of the allograft.
Xenograft materials, i.e., a graft of material transplanted between animals of different species, are readily available but are potentially the most antigenic and immunogenic of all implantable substitutes.
Reduction of the immunogenicity and/or antigenicity of xenografts as well as any other implant material is desirable because the shortage of suitable implantable tissue would be reduced or alleviated. For example, the supply of replacement tissue would be increased tremendously if bovine tendons and ligaments were used as substitutes for human tendons and ligaments. Fresh tendon allografts or xenografts elicit a potent immune response, stimulating cytotoxic antibodies and cell-mediated immunity. The rejection response is due to cellular antigens and not the collagen matrix which serves as the tissue's backbone structure. Consequently, removal or blockage of cellular antigens or entire cells will thus reduce immunogenicity and pave the way for an increased stock of implantable materials.
Removal of antigens should be accomplished in such a way as to not disturb the integrity of the collagen matrix. Damage to the matrix decreases the mechanical strength of the implant. Structural alterations in the collagen fibers may also lead to an inability of the graft to be remodelled, revascularized and re-integrated. Early attempts at cellular extraction involving a procedure using a two-phase organic extraction, i.e., chloroform/methanol and hydrochloric acid were successful but severely damaged the underlying collagen matrix. Even when the hydrochloric acid was eliminated and an organic extraction involving chloroform/methanol followed by phosphate buffer was used, the mechanical strength of bovine tendon was found to be 25% of pretreatment levels prior to implantation in a host. After implantation, tissues de-antigenized by this process do regain some mechanical strength.
Another disadvantage of the chloroform/methanol extraction procedure results from disposal of the chloroform waste. Chloroform is toxic and must properly be disposed of. Such disposal is costly, but regardless of cost, will always represent a source of pollution in the earth's ecosystem. Moreover, the use of chloroform is problematic both in the work place and potentially as a contaminant in the processed tissue.
Another method for reducing antigenicity of allografts or xenografts involves glutaraldehyde fixation, Glutaraldehyde fixation is used commercially to prepare bovine heart valves for human implantation. However, such implants are known to retain some antigenicity after implantation. Glutaraldehyde processing is also known to cause an excessive inflammatory response in the surrounding host tissue following implantation. Moreover, the crosslinking resulting from the aldehyde produces a graft that is a permanent implant with little possibility of host cellular infiltration and remodeling.
Consequently, there exits a need to remove cellular debris and reduce antigenicity by less toxic and more efficacious means. A new process is disclosed herein which alleviates or removes the above-described shortcomings in debris and antigen removal from implantable tissue.